The Rotary-wing X/C, Part II: Weather and Other Concerns

While helicopters obey the same laws of physics as airplanes, there are pronounced differences when it comes to control and operation — same air, different machine — and many ingrained fixed-wing reactions can be catastrophic when applied to rotary-wings. And since close to three quarters of the roughly 25,000 pilots flying helicopters also fly airplanes, this issue is worth discussing. These differences apply across the board — to developing weather, too — not just to sudden unexpected events. Plus, some micro-scale aspects of helicopter flying are worth pointing out, many of which most new helicopter pilots might not consider during cross-country flying.

NIGHT FLIGHT… AND JUST PLAIN SEEING: Darkness brings the possibility of unexpected, and unpleasant, encounters. This is especially true in the low altitude regime dominated by helicopters — you don’t want to be too low, since you might not see that unlighted wire, ridge, or tower. Plus, helicopter flying itself requires an increased situational awareness simply to maintain control — and this means good visibility. A moment’s inattention, especially at night, and it would be all too easy to become disoriented. Actually, Robinson Helicopter requires that, at night, the pilot has either adequate celestial illumination or enough lights on the ground for visual reference. Due to the inherent instability of helicopters — it’s just too easy to lose control!

Example

: Without outside visual references, as you wondered which way was up, you could ‘

float the rotor

‘. Since airplane pilots are used to always having aileron control, a low-g rollover for example might be countered ‘

instinctively

‘ by opposite cyclic. Applying aft cyclic, to load the rotor disc again, would be the correct response here.

Inside Information: The number one cause of accidents for rotorcraft is collision with obstructions. Most corporate helicopter flights are done VFR (about nine out of 10 in fact, from a Business & Commercial Aviation magazine survey).

CEILINGS: At uncontrolled airports, helicopters use about a 500-foot pattern altitude. For instrument approaches, that transition area floor could be just 200 feet higher. If it’s a day where the ceiling is 600 broken, any airplanes will be IFR and could pop out right in front of you. (The responsibility to see and avoid incoming traffic in a safe manner rests entirely with you.) Obviously, extra caution is essential, radio communication is critical, and local topographic as well as airspace knowledge is very helpful.

WINDMILLS IN THE WIND: Helicopters make their own wind. In winter, when there’s loose powdery snow, one can actually create self-induced whiteout just by flying very low and slow — now imagine it during departure or landing… In a helicopter, your landing options are greater, but you still have to be careful! Helicopters operate at lower altitudes, where mechanical turbulence (read: wind moving around ridges, buildings and trees) is greater. Plus, helicopters frequently operate away from airports, where people aren’t as aware of safety concerns.

Danger

: If the aircraft is on the ground with the rotor spinning at low rpm and there’s a sudden gust of wind, that can cause the main rotor blades to flap down — an event of special significance for an inattentive passenger standing nearby.

APPROACHES: A solo pilot (typically in the right seat) doing a slope landing with a left crosswind blowing down the slope could easily be flirting with dynamic rollover. Another danger is landing in a confined area surrounded by tall trees: If a steady breeze is blowing, watch out! As ground speed drops, passing through treetop height into a ‘wind shadow‘, may cause you to lose that headwind, and send you into those trees. In a small helicopter, ‘effective translational lift‘ kicks in at about 10-15 knots. The skinny is this: Once you lose that lift, if you’re already descending quickly enough, you could sink into and through your own downwash … and the ground (except for the ‘through‘ part). Another manifestation of this occurs on downwind approaches. An airplane pilot might willingly accept a five-knot tailwind for an up-slope landing, but in a light helicopter, especially if a steep approach is involved, don’t go near it — you may be over-run by your own downwash.

TAXIING… AND JUST SITTING THERE: Despite the natural weathervaning tendency of the fuselage in a slipstream of air, most helicopters are fighting a continual battle to overcome the torque of the main rotor. Danger: A hovering helicopter in the low-altitude/low airspeed flight regime isn’t ‘anchored‘ by its gear, and a phenomenon called ‘LTE‘ (loss of tail rotor effectiveness) can occur. The basic culprit is differing rates of turn for a given pedal position under varying wind conditions, and uncommanded rapid yaw rates. The result can be loss of aircraft control. It has happened even to the larger civil and military helicopters (Advisory Circular 90-95, 12/26/95). Basically, if you avoid downwind maneuvering, large left crosswinds (where the tail rotor itself can enter a vortex ring state as its blast is blown back upon itself), and squirrely winds, you’re better off.

STALLS: One scenario not normally thought of in a fixed-wing aircraft is the likelihood of a stall while established in stable, level flight at high speed. Retreating blade stall, poses the highest risk when you combine high density altitude, maximum gross weight, bottom-of-the-green RPM… and then throw in flight near Vne. Consider this. The faster the helicopter goes, the slower its main rotor blades travel through the air on the ‘retreating‘ side of their rotational plane. At some forward speed for the helicopter, its blades will no longer be able to produce lift on the retreating side. But there’s more…

Example

: Buzzing along over a large parking lot, and then crossing over a lake, you could encounter a sudden downdraft, the resultant control corrections could cause the critical angle of attack to be exceeded at a speed much lower than you would expect. (This actually happened to a JetRanger pilot over Boston.)

JUGGLING: As those of you experiencing solo cross-country flight in a small helicopter have no doubt discovered, CRM is absolutely critical. If it’s summertime, the doors are off, and you’re solo, you will discover that you now have just ‘half a hand‘ (or as I said last week, you need a third one). Forget about folding sectionals in flight! And back near the ground, if you have allergies and you suddenly have a sneezing attack in the proximity of other aircraft, people, or structures, trouble awaits.

RAMP SNAKES: Another low-level consideration airplane pilots wouldn’t think about is being sucked back into the ground! Even though helicopters can land almost anywhere, we pay close attention to where we do land. If the ground is soggy, or if the tarmac is hot, both present the possibility of dynamic rollover on departure if one skid picks up, but Mother Earth doesn’t want to let go of the other one! The result is usually one balled-up helicopter.

RAIN: In a helicopter, I get nervous when it starts raining. Did you ever stick your hand out of a car window at highway speed, in the rain? It stings! As you might recall, energy increases with the square of the velocity. The advancing rotor blade tip on an R22 in cruise is doing 550 and then some. What’s the square of the ratio of 550-to-55? It’s 100, and it’s enough to take the paint right off those blades! Besides the paint issue, added power is required to overcome impacts with the rain, and of course in a piston helicopter, you’d have the carburetor heat on, and that saps power, also. When there’s ice, it’ll form on thinner airfoils (and other protrusions) first. Guess what?

FOGGED UP: Humidity is another critical area. If you aren’t watching that carburetor temperature gauge and wind up with carburetor ice, you could have about one second to enter autorotation or risk catastrophic rotor blade stall.

HIGH DOWN LOW: Paradoxically, the closer to the ground you are in a helicopter, the more aware you should be of density altitude. Airplanes down low generally can just add power, and go around, but helicopters are power-limited; for a small helicopter, the difference between power available and power required is much less. This is particularly critical in high density altitude, confined area operations. You can easily exceed In Ground Effect (IGE) hover maximums due to excess weight and high density altitude. For example, a flight in Southeastern California on a hot day in late August could tax the engine too much… as well as the occupants. Look before you leap; check operating limitations!

BOTTOM LINE: As it turns out, the price we must pay to enjoy the benefits of the world’s ultimate off-road vehicle is not just monetary. We must be mindful of not only the larger scale kind of weather, but micro-scale phenomena, topographic effects, surface characteristics, and sometimes, some things you just won’t find in an aviation weather forecast.